Thermotropic polyesters, exhibiting therefore optical anisotropy in the melt phase, are well known products and broadly described in literature, e.g. in British Polymer Journal (Dec. 1980, p. 154, "Liquid Crystal Polymer"); Journal of Macromolecular Science (1984, p.1705, "Liquid Crystalline aromatic polyesters"); Die Angewandte Makromolecular Chemie (19-82, 109-110, p.1, "Rigid Chain Polymer;")
By extruding such polyesters in the molten state, high tenacity fibres and moulded articles with outstanding stiffness, toughness and strength are obtainable.
Polymers with the above cited properties are generally easily melt processable and show high heat and oxidation resistance; furthermore, due to their crystallinity, also the HDT (Heat Distortion Temperature) and the resistance to solvents of such polymers is high.
Thermotropic liquid crystalline polyesters with the above mentioned characteristics and suitable for producing fibres or films are described in U.S. Pat. No. 4,447,593. Such polyesters are obtained by polymerizing aromatic bicarboxylic acids with aromatic diols, the 90% of which are hydroquinones substituted with an alkyl group having at least 5 atoms of carbon or aralkyl group having at least 7 atoms of carbon.
The applicant is acquainted with other prior art documents in which aromatic bicarboxylic acids are used together with hydroquinones or variously substituted hydroquinones.
European Patent Application No. 295,892 refers to thermotropic liquid crystalline polymers in which the units of aromatic diols are phenylhydroquinone and .alpha.-methylphenylethylhydroquinone.
European Patent Application No. 320,298 refers to thermotropic liquid crystalline polymers in which the units of aromatic diols are (1-phenylethyl)hydroquinone and .alpha.-methylphenylethyl-hydroquinone.
U.S. Pat. No. 4,668,760 refers to thermotropic liquid crystalline polymers in which the units of aromatic diols are phenyl-hydroquinone and (1-phenylethyl)-hydroquinone.
U.S. Pat. No. 4,614,791 refers to liquid crystalline polymers in which the units of aromatic diols are (1-phenylethyl)hydroquinone and hydroquinone. Hydroquinone units are in mole percentage of from 16% and 18% of the total content of aromatic diols. In such document it is maintained that when the mole percentage of (1-phenylethyl)hydroquinone (based on the total moles of the reacting bicarboxylic acid and diols) is less than 41%-42%, an intractable material is formed.
U.S. Pat. No. 4,360,658 refers to copolyesters in which aromatic diols are constituted from 25% to 50% of hydroquinone and from 50% to 75% of phenylhydroquinone.